1. Field of the Invention
This invention relates to a pressure sensitive adhesive composition which has good adhesive characteristics and good heat resistance and weatherability.
2. Description of the Related Art
From the standpoint of energy and resource savings and pollution control, hot melt-type adhesives have been widely employed instead of solvent-type adhesives in which natural rubber and the like are dissolved in organic solvent. As a typical base polymer of the hot melt-type adhesives there have been used block copolymers comprised of aromatic vinyl block-conjugated diene block-aromatic vinyl block (U.S. Pat. Nos. 3,676,202 and 3,723,170). These copolymers are good with respect to adhesion at normal and low temperatures and flexibility. However, since the copolymers contain the diene block which is unsaturated in nature, they suffer considerable deterioration upon exposure to ultraviolet rays or heat and cannot be used under these circumstances. In order to overcome the drawback of the copolymers, there is known a method wherein the conjugated diene moiety is hydrogenated to reduce the amount of the unsaturated bonds (U.S. Pat. No. 3,427,269). However, the copolymer whose conjugated moiety has been hydrogenated tends to lower in adhesive characteristics including not only adhesion at low temperature, but also adhesion at normal temperatures and adhesion to polyolefins when compared with the non-hydrogenated copolymers. In addition, such hydrogenated copolymer exhibits so a high melt viscosity at high temperatures that processability undesirably lowers. As a result, severe limitation is placed on the use from the standpoint of adhesive characteristics and processability. In order to solve these problems, plasticizers or softeners such as process oils have been in use as described in "Rubber Chem. Technol., 29 492 (1956)". Although adhesive characteristics at low and normal temperatures and processability at high temperatures are improved by incorporation of these additives, adhesion to polyolefins is not improved at all with a considerable lowering of cohesion at relatively high temperatures. Moreover, the oils added sometimes bleed out from the adhesive, thus causing the substrate for the adhesive and adherend to be contaminated therewith. This will eventually bring about the problem that the adhesive greatly changes its characteristics with time. For these reasons, the adhesives of the type set out above are limited in the range of applications.